An organic electroluminescent element has been known in general, which is hereinafter also referred to as an “organic EL element”. The organic EL element has a structure in which functional layers like a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, and an electron injection layer are stacked between a cathode and an anode disposed on a substrate. In the organic EL element, voltage is applied between the anode and the cathode, where by planate light is emitted outward from the emission layer.
In the organic EL element, light is emitted outward through an optically transparent electrode. There has been known a light out-coupling structure where an electrode paired with the optically transparent electrode is composed of a reflective electrode so that light from the emission layer is reflected by the reflective electrode to be emitted outward. By using the reflective electrode, it is possible to efficiently emit the light outward.
In the organic EL element, light out-coupling efficiency is important, and a structure to improve light out-coupling property more has been developed. In Japanese Patent Publication Number 2003-123987, a technique for obtaining a high brightness is disclosed, in which an emission layer is disposed between a total reflection mirror and a half mirror, thereby forming a resonant structure. However, in this structure, it is difficult to adjust a thickness of each layer, and it may be hard to form the organic EL element.
Further, as the structure for improving light reflectivity, it is conceivable to increase reflective index of the reflective electrode. For example, the reflectivity of the electrode can be expected to be improved by using silver as an electrode material because silver is a metal having high reflectivity.
It is not however easy to improve electrical reliability of the electrode containing silver because silver has property to be stacked to sneak during film formation, thereby facilitating the occurrence of short circuit when a foreign matter is mixed. Also, using a lot of silver in order to improve reflectivity can cause high-cost and decreasing of economic efficiency